1. Field of the Invention
This invention relates to a process of producing sponge iron by a direct reduction of iron oxide-containing materials comprising a prereduction if a fluidized bed and a final reduction carried out below the melting point of the charge in a rotary kiln.
2. Description of Prior Art
In the dressing of relatively poor iron ores, fine-grained concentrates having a substantial proportion of particles smaller than 0.25 mm become increasingly available. These concentrates and fine-grained ores are less expensive than, e.g., pellets or lump ores.
On the other hand it is difficult to subject such fine-grained materials which contain iron oxide to a direct reduction in a rotary kiln because fine-grained material and particularly the very small particles contained therein in a high proportion tend to form agglomerates and crusts in the rotary kiln.
It is known from German Offenlegungsschrift No. 20 20 306 to charge a rotary kiln with fine-grained ores having a particle size of about 0.25 to 3 mm and with sulfur-binding materials having a particle size of about 0.2 to 2 mm, to drive the rotary kiln at a peripheral velocity of 2 to 20 meters per minute and to maintain in the reduction zone a temperature between 1000.degree. C. and 1115.degree. C. This practice imposes a lower limit regarding the particle size of the ore and requires the rotary kiln to be rotated at a higher speed as the fines content increases.
It is known from German Offenlegungsschrift No. 15 33 869 to effect the direct reduction with reducing gases in two stages in order to improve the utilization of the gases. One-half of the total combined oxygen contained in the ore is removed in the prereduction stage and the other half in the final reduction stage and a metallization of about 25 to 35% is effected by the prereduction. The prereduction may be effected by a countercurrent operation in a shaft furnace, a rotary kiln or a fluidized bed. The final reduction can also be effected in such equipment in a countercurrent cocurrent or transverse current operation. The use of rotary kilns and the processing of fine-grained ore involve the disadvantages described hereinbefore. Even if a fluidized bed is used for the prereduction, the disadvantages encountered in a succeeding rotary kiln cannot be avoided.
It is also known to carry out in a fluidized bed a prereduction resulting in a metallization of 50 to 80% and to subject a molten charge to the final reduction in an electric furnace (Offenlegungsschrifts Nos. 25 52 904 and 26 07 554, German Pat. No. 22 53 228). Expensive electric energy is consumed at a hihg rate for the final reduction. A metallization to a higher degree in the fluidized bed involves difficulties in the fluidized bed.